docs: def write(self, config_file):
          """
          Write a single config file, raises if config file is not registered.
          """
          if config_file not in self.templates:
              log('Config not registered: %s' % config_file, level=ERROR)
              raise OSConfigException
  
          _out = self.render(config_file)
          if six.PY3:
              _out = _out.encode('UTF-8')
  
          with open(config_file, 'wb') as out:
              out.write(_out)
  
          log('Wrote template %s.' % config_file, level=INFO)